Information provided by a grid monitoring system is generally used to supervise the grid operating conditions, diagnose grid faults characteristics, synchronize power converters to the grid, calculate power flows, translate state variables of complex systems into synchronous reference frames, and calculate current and voltage references of power converters operating under generic grid conditions.
Regardless of the technique used in the monitoring system, amplitude, frequency and phase of the monitored variables must be obtained in a fast and accurate manner. Additionally, when the monitoring system is applied to a multi-phase networks preferable three phase grid, sequence components should be rapidly and precisely detected as well, even if the utility voltage is distorted and unbalanced. It is worth to remark that a real-time monitoring system should be based on a simple algorithm, with a low computational burden, which can be computed in each sampling period.
Grid monitoring can be performed by assuming a constant and well know value for the utility frequency. This approach makes calculations simple but gives rise to detection errors when frequency differs from its rated value. It is a common incident in micro-grid and weak grids, with widespread usage of distributed generators, working under faulty conditions. The most extended technique used for achieving a frequency-insensitive grid monitoring is using a Phase-Locked Loop (PLL). Regarding three-phase systems, considered as the most general case of grid monitoring, the following real-time monitoring systems based on the usage of some kind of PLL has been reported in the literature:
Monitors based on the Synchronous Reference Frame (SRF-PLL) [1] are broadly used in measurement and control systems. However, as reported in the literature [2], the SRF-PLL gives rise to unacceptably deficient results in presence of the voltage imbalance.
Monitors based on the Decoupled Double Synchronous Reference Frame (DSRF-PLL) [3] are able to estimate the positive- and negative-sequence components at fundamental frequency under unbalanced grid conditions. However the DSRF-PLL should be implemented in a high performance digital signal processor because of its notable computational cost. This constraint complicates its application in the detection of multiple harmonic components.
Monitors based on the single-phase Enhanced Phase-Locked Loop (EPLL) [4] achieve the isolation of the positive- and negative-sequence components by means of the usage of frequency adaptive single-phase notch filters [5] and the instantaneous symmetrical components method on the a-b-c reference frame [6]. The EPLL-based GMS could be applied under unbalanced grid conditions, however its response is slow and its computational burden is so high since four singe-phase EPLLs are necessary to implement a three-phase application. Moreover, its single-phase origin makes the GMS sensible to the effects of zero-sequence components in the grid voltage.
Monitors based on the Dual Second Order Generalized Integrator (DSOGI-PLL) [7] are also able to estimate the positive- and negative-sequence components under unbalanced grid conditions and its computational cost is smaller than earlier approaches.
All the previously presented grid monitoring techniques come from the evolution of the conventional single-phase PLL technique and different approaches have applied in order to make them robust enough for detecting sequence components in three-phase systems under generic unbalanced and distorted grid conditions. Consequently, all of these techniques have a common characteristic, namely, the monitoring algorithm is based on the detection of the phase-angle of the variable to be screened.
WO 02/091578 concerns a new structure for phase-locked loop (PLL) system. As with conventional PLLs, this invention consists of phase detection, loop filter and voltage-controlled oscillator units. An alternative phase detection structure, inspired by concepts from adaptive filtering and dynamical systems theory, is presented which substantially enhances the performance of the loop in terms of stability and dynamic performance. Presented phase detection scheme obviates the need for sophisticated loop filters, so much so that a first order filter suffices for most applications. In addition to the normal function of a PLL, the present system directly generates estimates of the amplitude, phase and frequency of the input signal. This feature extends the range of applications of the system beyond well-known applications of PLL in various disciplines of electrical.
US 2005/0231871 concerns a Three-phase Power Signal Processor (TPSP) is disclosed for general three-phase power system applications. The TPSP is developed based on the concepts from adaptive filter and dynamical systems theories. The structure of the TPSP is unified as it provides a multiplicity of the signals and pieces of information without the need to change, modify, or enhance the structure or to impose excessive computational time or resource requirements. The presented TPSP receives a set of three-phase measured signals, which can be voltage, current, magnetic flux, etc, and provides (1) the instantaneous and steady-state symmetrical components, (2) the fundamental components, (3) the peak values (magnitudes) of the symmetrical components, (4) the frequency and its rate of change, (5) the synchronization signal(s) and zero-crossing instants, (6) the phase-angles of the symmetrical components, and (7) the disturbance signatures. Two or more TPSP units, when properly augmented, further provide (8) the individual harmonic components, (9) the inter-harmonics, (10) the instantaneous real and reactive current components, (11) the total harmonic distortion, dc-offset, and power factor. The TPSP can serve as the building block for various signal processing requirements encountered in the context of power system applications including power systems control, protection, monitoring, and power quality.